Immiscible liquid phases can often be separated simply by gravity settling when each phase has a different density. Continuous separation based on this principle is accomplished by a "decanter" as is well known in the art.
Porous membranes have been used for liquid/liquid phase separation, see for example, Nord et al., Analytica Chimica Acta, Vol. 118, 1980, pages 285-292. When one phase wets the porous membrane and the other phase does not, then the membrane wetting phase can pass through the pores of the membrane when driven by hydraulic pressure. However, the phase that does not wet the membrane theoretically is not driven through the pores of the membrane under pressure when the pressure is less than the "crucial pressure." The "crucial pressure" is a function of pore diameter (r), the contact angle of the nonwetting phase on the surface of the membrane (.theta.) of the surface tension (.tau.) of the nonwetting phase as theoretically described in the following equation (reference, W. J. Moore, Physical Chemistry, 1964, Prentice-Hall, p. 730). EQU "crucial pressure"=(2.tau. cos .theta./r)
A liquid phase that wets a membrane has a negative "crutial pressure."
As a practical matter, membrane phase separators suffer problems of "durability" as described by Nord et al., supra, i.e., eventual passage of undesirable mixed phases through the membrane. The membrane phase separation systems of Nord et al. were considered to have durable performance if they worked longer than two hours.
Among the causes of the durability problem with membrane phase separators is a change in the wetting characteristics of the membrane with use. A new hydrophobic membrane (not wetted by water) may absorb water in time or may adsorb onto its surfaces contaminants that lower the "crucial pressure." Additionally, fluctuations in the surface tension of the phases can cause intermittent failures of a membrane phase separator to completely separate one phase from a mixture of phases.
The present inventor was frustrated by the durability problems of prior membrane phase separation systems used in liquid/liquid segmented flow analytical chemistry applications even when the problem was only occasional or intermittent.
Thus, it is an object of this invention to provide a continuous membrane phase separation process effective to separate at least a portion of one phase from a liquid/liquid phase mixture, said separated phase not containing other liquid phases, said process operable for an extended period of time in excess of eight hours.
Essential to the invention is the staged use of porous membranes. It is well known in the art of material processing to use staged nonporous membranes as generally described in Membranes in Separations, Hwang and Kammermeyer, 1984, R. E. Krieger Publishing Co., Malabar, Fla., ISBN 0-89874-801-1, pages 325-343. However, little work has been done with staged porous membranes. Pall in U.S. Pat. No. 4,431,545 describes the use of staged porous membranes for a filtration process but without an output port connected to the space between the membranes. Fossey et al. in Anal. Chem., Vol. 55, 1983, pp. 1882-1885 describe a dual porous membrane apparatus for simultaneous monitoring of both phases in the solvent extraction/flow injection analysis of Dramamine.RTM. tablets. However, in the apparatus of Fossey et al. the mixed phase is fed to the space between the membranes and one membrane is hydrophobic while the other membrane is hydrophilic.